1. Field of the Invention
The present invention relates to an improved process for manufacturing 5-hydroxyisophthalic acid (hereinafter referred to as "5-HIPA"), which is useful as a raw material for producing high-performance polymers, pharmaceuticals and agrochemicals.
2. Description of the Related Art
It is well known that 5-HIPA can be produced by alkali fusion of 5-sulfoisophthalic acid. Nevertheless, this method poses major technical problems, for example, large amounts of fuming sulfuric acid or alkaline material such as caustic soda have to be used, the sulfonation reaction affords only poor selectivity, large amounts of by-products are produced in the alkali fusion step, and the reaction is required to proceed at a high temperature as well as under high pressure, thus rendering it quite difficult to inexpensively produce high-purity 5-HIPA in a large quantity.
It is also public knowledge that 5-HIPA can be produced by oxidizing 5-acyloxy-m-xylene with molecular oxygen in the presence of lower aliphatic carboxylic acid and acetic anhydride, using a cobalt compound and a bromine compound, or a cobalt compound, a manganese compound and a bromine compound. (Japanese Patent Publication No. Sho-57 (1982)-15737.) Since the above-mentioned method is, however, an oxidation reaction carried out under an elevated pressure of 20-50 kg./cm.sup.2 gauge, using either oxygen or air, the 5-acyloxy- isophthalic acid thus produced exhibits a poor color. That is to say, the product is discolored in that it is yellowish brown or gray, and the method by itself gives an insufficient yield. Even if such discolored 5-acyloxyisophthalic acid is hydrolyzed, there cannot be obtained 5-HIPA that can be utilized as the raw material for high performance polymers, pharmaceuticals or agrochemicals.
The present inventors have arrived at the present invention by discovering means by which to resolve the above-mentioned problems after intensive study.
In fact, the present inventors have discovered that the above-mentioned problems can be resolved by oxidizing with air 5-acyloxy-m-xylene at a pressure of 2-15 kg./cm.sup.2 gauge which is lower than the conventionally adopted pressure level in the presence of lower aliphatic carboxylic acid and acetic anhydride, with a catalyst system consisting of a cobalt compound and a bromine compound, while letting a specific amount of alkali metal compound coexist with the reaction system.
Hence, the object of the present invention is to provide a method for producing at a high yield and more inexpensively, 5-HIPA that can be utilized as the raw material for high-performance polymers, pharmaceuticals or agrochemicals by means of air-oxidizing 5-acyloxy-m-xylene in the liquid phase.